Before a wireless device can provide a user with wireless functionality such as telephony and messaging, the wireless device registers with a wireless network. There may be a plurality of available wireless networks in a given geographic coverage area. Therefore, the wireless device selects one of the available wireless networks for registration. After registering with the selected network, the wireless device might operate in an idle mode where it camps on a communication channel of the wireless network to monitor for calls or messages, or initiate communication.
Wireless network selection typically involves the wireless device selecting the wireless network to which the wireless device was last registered, if available. However, if the last registered network is unavailable, which might be the case if the wireless device has moved to another geographic coverage area, then the wireless device selects another wireless network. The wireless device might have a SIM (Subscriber Identity Module) containing information identifying high priority networks including a home network and preferred roaming networks. Therefore, when the last registered network is unavailable, the wireless device can select another wireless network according to the information stored in the SIM.
The information stored in the SIM might be limited to a geographical region such as a country or a set of countries. This is because the preferred roaming networks depend on various roaming agreements. Therefore, in some situations when a user travels to another country, the wireless device might not be able to find the home network or any of the preferred roaming networks indicated in the SIM. Train travel with the wireless device can trigger this problem. In this situation, the wireless device might randomly select an available wireless network for registration from those available wireless networks that offer strong RSSI (Received Signal Strength Indication) such as RSSI≧−85 dBm for GSM (Global System for Mobile communications) and RSSI≧−95 dBm for UMTS (Universal Mobile Telecommunications System). If there are no wireless networks that offer strong RSSI, then the wireless device might select the wireless network that offers the strongest RSSI. Unfortunately, the wireless network selected in this manner might not be preferred over other available wireless networks in terms of service offered. For instance, the wireless network selected might not offer CS (Circuit Switched) service together with PS (Packet Switched) service for the wireless device.
The wireless device might have no way of detecting preferred networks. For example, if the wireless device is in automatic mode with a SIM that does not have a PLMN (Public Land Mobile Network) selector to identify preferred roaming networks, then the wireless device will have no way of selecting a preferred PLMN. To address this problem, a home carrier network can employ a managed roamer box. A registration request from the wireless device is routed through a foreign network back to the home network. Upon determining that the foreign network is not preferred, the managed roamer box of the home network sends a CS attach reject code 17 to the wireless device via the foreign network. Upon receiving the rejection, the wireless device will make three further registration requests and if these are rejected with the same cause then the wireless device might attempt to register with other wireless networks and might be able to register with a preferred network. The wireless device might make several attempts to register with several different networks on an on-going basis. The managed roamer box will eventually allow registration with a foreign network that is not preferred after five registration attempts.
The managed roamer box might not reject PS services in some networks such as NMO2 (Network Mode of Operation 2) networks or NMO1 (Network Mode of Operation 1) networks. Therefore, upon registering with the wireless network that is not preferred, the wireless device might be PS attached while at the same time not being CS attached. Therefore, the wireless device might be able to establish an active PDP (Packet Data Protocol) context for use in data communication, but will not be reachable via CS calls or SMS. The wireless device might indicate to the user that a GPRS (General Packet Radio Service) network has been acquired, yet the user might be unaware that they are not reachable via CS voice. Furthermore, in some cases as per standards, there is no network reselection trigger. Therefore, the inability to receive a CS voice call or SMS continues. This can be undesirable for the user of the wireless device.